Interference in Thin Films

Optical lenses (n=1.40) in cameras are coated with a thin film of magnesium fluoride (n=1.38) to reduce reflections. What minimum thickness is required to give minimum reflection at a wavelength of 550 nm?

2. Relevant equations

2nt = (lamda)/2

3. The attempt at a solution

I assume the relevant equation is what I'm supposed to use. so t=(lamda)/(4n)
Therefore, the thickness of the film, t=550/(4*1.38)=99.6 nm

I wasn't given the answer, so I'm not sure if this is correct. I wasn't sure what if the index of refraction of the camera was irrelevant information.